High molecular weight linear polyester resins of the poly(1,4-butylene terephthalate) family are known to be uniquely superior as components in thermoplastic molding compositions, because of their excellent physical properties and surface appearance after molding. In comparison with lower homologous polyesters, such as poly(ethylene terephthalate) and poly(1,3-propylene terephthalate), the poly(1,4-butylene terephthalate) resins crystallize very rapidly from the melt and therefore they can be formulated into compositions which are moldable in conventional equipment with conventional temperature and cycle times, and without the need to use nucleating agents.
Methods for the preparation of poly(alkylene terephthalates) by direct esterification of terephthalic acid (TPA) with the corresponding alkylene glycols are well known.
Most known methods employ high temperatures and superatmospheric pressure conditions to promote solubilization and reaction of the intractable TPA.
When these known conditions are applied to the preparation of poly(1,4-butylene terephthalate) (PBT) resins, excessive amounts of 1,4-butanediol are required due to dehydration of part of the butanediol to tetrahydrofuran. This side reaction makes the known direct esterification methods economically unattractive for the preparation of PBT. For example, according to a recently published method (Netherlands Pat. No. 7105777 (1971)), the butanediol usage was 376% over the theoretical amount required.
It has now been discovered that the side reaction, causing the use of such excessive amounts of 1,4-butanediol, can be almost totally suppressed if reaction conditions are selected which promote the rapid removal from the reaction zone of byproduct water (formed in the esterification reaction), and which minimize the time spent by the 1,4-butanediol in contact with unreacted TPA.
The following significant parameters are to be controlled:
A. To promote rapid removal of water formed: PA0 B. To minimize contact between 1,4-butanediol and TPA:
1. A reaction temperature in the range of from about 175.degree. to about 275.degree. C. PA1 2. Atmospheric or subatmospheric pressure immediately from the start of the reaction. PA1 3. Presence of a high boiling solvent, e.g., tetralin, decalin, and the like, or preferably excess 1,4-butanediol, to entrain and help drive off water. PA1 4. Gradual addition of the TPA to the hot 1,4-butanediol, facilitating the rapid removal of water as formed. PA1 1. Use of an efficient catalyst, such as an antimony compound, and the like, but preferably a titanium compound or a tin compound to shorten the reaction time. PA1 2. Gradual addition of TPA and a high reaction temperature as mentioned under A. PA1 (a) heating 1,4-butanediol to a temperature in the range of from about 175.degree. to about 275.degree. C.; PA1 (b) gradually adding terephthalic acid to the hot 1,4-butanediol; and PA1 (c) removing the byproduct water from the reaction mixture as the water is formed. PA1 (a) heating a mixture of terephthalic acid and at least 1.25 moles of 1,4-butanediol per mole of the acid at a temperature in the range of from about 175.degree. to about 275.degree. C. at atmospheric or subatmospheric pressure; PA1 (b) distilling the excess 1,4-butanediol from the reaction mixture; and PA1 (c) removing the byproduct water by entrainment in excess 1,4-butanediol distilled from the mixture.
The poly(1,4-butylene terephthalate) resins can be obtained in very economical fashion by the present improved process. Moreover, the products may be just as easily compounded and will ultimately provide molded articles with substantially the same superior properties as those made from the best of the prior art polyesters.